"Wherefore the human soul derives its intellectual light from Him, according to Ps. 4:7, 'The light of Thy countenance, O Lord, is signed upon us.'"

Metaphor in Context

Wherefore some held that this intellect, substantially separate, is the active intellect, which by lighting up the phantasms as it were, makes them to be actually intelligible. But, even supposing the existence of such a separate active intellect, it would still be necessary to assign to the human soul some power participating in that superior intellect, by which power the human soul makes things actually intelligible. Just as in other perfect natural things, besides the universal active causes, each one is endowed with its proper powers derived from those universal causes: for the sun alone does not generate man; but in man is the power of begetting man: and in like manner with other perfect animals. Now among these lower things nothing is more perfect than the human soul. Wherefore we must say that in the soul is some power derived from a higher intellect, whereby it is able to light up the phantasms. And we know this by experience, since we perceive that we abstract universal forms from their particular conditions, which is to make them actually intelligible. Now no action belongs to anything except through some principle formally inherent therein; as we have said above of the passive intellect (Q76, A1). Therefore the power which is the principle of this action must be something in the soul. For this reason Aristotle (De Anima iii, 5) compared the active intellect to light, which is something received into the air: while Plato compared the separate intellect impressing the soul to the sun, as Themistius says in his commentary on De Anima iii. But the separate intellect, according to the teaching of our faith, is God Himself, Who is the soul's Creator, and only beatitude; as will be shown later on (Q90, A3; FS, Q3, A7). Wherefore the human soul derives its intellectual light from Him, according to Ps. 4:7, "The light of Thy countenance, O Lord, is signed upon us."
(I, Q79, A4)